18Watt.com

I want to try the Paul Ruby buzz fix on my 36W (Ceriatone) but I am a bit confused by the zener diode. The article list a 1N575A but that seems to be an obsolete part number. I did find a data sheet and the zener is 9.1V .5W which would be a 1N5239 at Mouser so I am assuning that is OK. A 9.1V 1W is a 1N4739.

The buzz fix article is for an 18 watter and the circuit is for a single EL84. In a 36 watter the cicuit would be applied to two tubes so should I use a 1W zener diode? I believe that a zener diode should be run at 10% to 80% of it's rated power. I ordered .5W and 1W zeners, they're cheap.

How do I check this circuit when I get it installed?

Thanks in advance to anyone who replies.

Soler on,
George

From what I've just read the zener diode should be rated about 2V higher than your cathode voltage ... in the Paul Ruby document he had 7.5V on the cathodes with the 'old-style' 120 ohm cathode resistor and used 9.1V zener diodes ...

Thanks to rock_mumbles for the reply. After digging through the forum on this web site and at The Amp Garage I found the same information. My amp has 150R cathode resistors and 10.4V on the cathodes so I will buy 12V and 13V zener diodes to try. It also looks like I may be the type of person who would like a lower value grid leak resistors for less gain, maybe 220K.

Solder on,
George

I did some scope testing recently on an 18 watt TMB version and carefully tuned it so that the Zeners clipped on the bottom of the wave just after the grid current clipped the top. 16V, 1W Zeners did the trick and prevented the coupling caps from charging and shifting the bias point. Crossover distortion was dramatically reduced.

Thanks to Chuck H's suggestion, I also ended up with a 13V Zener across the 150 ohm bias resistor to prevent bias shift under heavy drive. This practically eliminated crossover distortion at even ridiculous amounts of overdrive.

I put both mods on unlabeled switches and let my friend I built the amp for choose which settings he liked best. He had never played a tube amp before but within a few minutes had both switches in the MOD position.

The result was the amp was noticeably louder with more of a mid focus and much less white noise at high gain levels. The mods only had an effect when clipping the outputs so the clean sound was unaffected.

..Joe L

Joe_L wrote:Thanks to Chuck H's suggestion, I also ended up with a 13V Zener across the 150 ohm bias resistor to prevent bias shift under heavy drive. This practically eliminated crossover distortion at even ridiculous amounts of overdrive.

Is that a thread here??? Could you link to it or explain more about this?

Colin_D wrote:

Joe_L wrote:
Thanks to Chuck H's suggestion, I also ended up with a 13V Zener across the 150 ohm bias resistor to prevent bias shift under heavy drive. This practically eliminated crossover distortion at even ridiculous amounts of overdrive.

Is that a thread here??? Could you link to it or explain more about this?

It's here: http://music-electronics-forum.com/t10540/#post82968

Is the process the same as the ruby fix? I.e you measure the voltage across the resistor and use a Zener rated 2v above that value?

Thanks

Bronsboi wrote:
Is the process the same as the ruby fix? I.e you measure the voltage across the resistor and use a Zener rated 2v above that value?

Thanks

Just clarify things, you are referring to the zener across the cathode resistor, correct?
For the cathode fix (guitar amp guys say) you'll use 5 watt zener(s) about 2 or 3 volts above the measured idle cathode voltage ...

Search on this page for zener:
http://www.tubecad.com/2007/12/blog0129.htm
he uses a zener about 3 to 7 V over the idle cathode voltage

Yep, I've done the ruby Zener fix and liked the results, I'm gonna try the Zener fix on the cathode resistr aswell, just to see if I like it. Thanks for the link Rock,

I see it's only been 7 months and I finally got around to trying this mod. It did make a difference, for the good. The overdrive sound is a bit smoother. The bass is a bit tighter at all levels.

I measured the cathode bias voltage and I got 11.0 and 11.1 volts so I used two 6V 5W zeners across the cathode bias resistors. I used 15V 1W zeners for the buzz fix.

This is my first amp and it still isn't right but it is much better than when I first built it. At first the bass was quite loose. Much to my surprise, replacing the 'Holy Grail' electrolytic bypass capacitors with BC Components capacitors was a noticable improvement.

Pictures of my 'Buzz Fiz'
http://imageshack.us/g/12/paulrubybuzzfix.jpg/

"This is my first amp and it still isn't right but it is much better than when I first built it."

That is the pattern I am afraid. It took me a year of fiddling to get my 18w trem "right". That included getting the right speakers as well as the internal mods. Now over the second year (anniversary just passed) I have not done anything other than play with various makes of valves. So you will get it to sound as you want it, I predict.

I wake up every day wanting to switch on and play through it. That never happened with bought amps.

Dd

The ruby fix worked for me, I use at 14v for full volume and a 4.5 v one for low VVR settings (both on a switch)
The zener across the cathode resistor just seemed to turn the volume down!...(not always a bad thing), I don't know if this is cos I've done the large bypass mod as well... but in the end I just removed the diode.
Dave

Daviedawg wrote: ↑

Tue 08/21/12 1:52 am

"This is my first amp and it still isn't right but it is much better than when I first built it."

That is the pattern I am afraid. It took me a year of fiddling to get my 18w trem "right". That included getting the right speakers as well as the internal mods. Now over the second year (anniversary just passed) I have not done anything other than play with various makes of valves. So you will get it to sound as you want it, I predict.

I wake up every day wanting to switch on and play through it. That never happened with bought amps.

Dd

Dd...

What did you mean with fiddling to get the trem right? What was wrong with it? You wouldn't happen to have a cure for the dreadfull omnipresent thumping? Both my 18 watts have a little backgroud thump. When you play it's not audible, but on idle it thumps somewhat. I thought it was inherent to the design...or isn't it?

Those comments pertain to the whole amp. The key to it was getting the voltages to the power valves right. But I did have issues getting the trem itself to work. But it did not take long to resolve. The voltage was the main "fiddle" but also I tried several different twin triodes before I got one that started oscillating immediately. The plate on the oscillator side of the triode is running at 189v.

The valve in place is a GE ECC83 which has been there for a long time.

I have not suffered from thumping with this amp. On both channels it is very quiet at rest (which was also part of the task of getting it right). It was my first big build from scratch so I got some layout wrong. But it is a totally reliable glorious sounding amp.

Dd